The sidebox header backgrounds are images called tile_back.gif.
If you want to remove that image for all sideboxes (and centerboxes), remove the red part:
.leftBoxHeading, .centerBoxHeading, .rightBoxHeading {
margin: 0em;
background-color: #abbbd3;
background-image: url(../images/tile_back.gif);
padding: 0.5em 0.2em;
}
Then you can change the green part to the color you want.
To change the sidebox background:
.sideBoxContent {
background-color: #ffffff;
padding: 0.4em;
}
If you want to change the boxes individually, you'll need to add statements to the stylesheet, using the proper class and id names, like #categories, #categoriesHeading, etc.
The easiest way to figure out what is where is by installing the Firefox browser and the Web Developer plugin for Firefox. You can use those to see where in the stylesheet you change the styling of the various page elements.
Ctrl-Shift-Y will allow you to hover over a section of the page and see the class and id names for that section in the box at the top. The statements in the stylesheet that begin with # are id's and the ones that begin with dots are classes.
Ctrl-Shift-E will open the CSS editor and allow you to edit the stylesheet and see the changes in real time without changing anything permanently. A very handy tool.